The present invention relates to a crankcase pressure regulator and, more particularly, to crankcase pressure regulation using airside heat transfer in the evaporator of a vapor-compression cooling system and the like.
As is well known in the relevant art, the basic vapor compression cooling or refrigeration system consists of a compressor, condenser, evaporator, and expansion (throttling) valve (typically a Thermal Expansion Valve, Capillary Tube, or Orifice Plate). In these systems, refrigerant is evaporated at low pressure to provide cooling and then this low-pressure vapor is then compressed by the compressor and condensed in the condenser, with heat being rejected at the higher condensation temperature. The condensed refrigerant is then throttled back to low pressure, to enter the evaporator and repeat the process.
Actual air conditioners, refrigeration systems and the like typically utilize other components in addition to these basic items, such as oil separators, suction-line (liquid) accumulators, liquid receivers, mufflers, recuperative heat exchangers, reversing valves, high pressure and low pressure safety switches, thermal overload protection, and filter-driers. The construction of and reasons for using these components are also well known in the art.
Essentially during startup or lower than design temperature lift, the capacity of the vapor-compression system increases dramatically causing the compressor to overload (capacity increases with decreased lift, i.e., the temperature difference between the condenser and evaporator). Therefore, in addition to the other components in a vapor compression cooling system, a crankcase pressure regulator is a common accessory that is added to many systems to prevent the compressor's motor from being overloaded when the pressure of the refrigerant supplied to the inlet of the compressor (the compressor suction side pressure) rises above the design pressure. As the inlet pressure to the compressor increases above the design pressure, a regulator is commonly employed (between the evaporator outlet and compressor inlet) to drop the pressure between the evaporator outlet and the compressor inlet.
Since the compressor crankcase is typically operated at the suction-side pressure, the above-mentioned valve is commonly referred to as a crankcase pressure regulator. These regulator devices are well know and commercially available (see, e.g., Sporlan Valve Company Bulletin 90-10, (January 1989). These regulator valves are also referred to as CRO valves, since their operation is to close on rise of the outlet pressure, i.e., they close on the rise of the compressor inlet pressure (the outlet of the CRO being connected to the compressor inlet) to regulate the compressor inlet pressure below a predetermined maximum pressure and thereby avoid compressor overloading.
Similarly, there is also another pressure regulating valve, that can be located between the evaporator outlet and the compressor inlet, known as an Evaporator Pressure Regulating valve to directly regulate evaporator, not compressor inlet, pressure. Such valves operate mechanically like a crankcase pressure regulating valve; however they control the pressure at the evaporator outlet rather than the compressor inlet. That, is rather than controlling the pressure at the regulating valve outlet (like a CRO), they control the pressure at the regulating valve inlet and are referred to ORI valves, since they open on rise of inlet pressure (or ORI). The performance of ORI valves is described in Sporlan Valve Company Bulletin 90-20 (January 1989). While the crankcase pressure regulators are designed to prevent compressor overloading, the ORI pressure regulating valves are used to keep the evaporator temperature from becoming too warm during operation at high evaporator heat loads, but do nothing to reduce compressor overloading (because at high evaporator temperatures they are fully open and provide no reduction in refrigerant flow).
The above-described crankcase pressure regulators are commonly spring activated mechanical pressure regulators. It is also possible, to utilize electrically actuated refrigerant valves controlled by the compressor inlet pressure to replace a CRO valve or controlled by the evaporator outlet pressure in the case of an ORI valve.
Another known method of maintaining the compressors suction line pressure from rising too much is to use a maximum operating pressure (MOP) thermal expansion valve (TXV), described at page 6 of Sporlan Valve Company Bulletin 10-9 (August 2005). The MOP TXV serves as another way to prevent compressor motor overloading. The MOP feature of the TXV causes the TXV to close above a predetermined evaporator pressure. By closing the TXV, the supply of refrigerant to the compressor is restricted, causing the suction line pressure to drop. Variations of this approach include the one described in U.S. Pat. No. 6,854,285, in which electrical feedback rather than fluidic feedback is used to control the expansion valve to maintain superheat and suction side pressure. Other feedback expansion device controllers are described in U.S. Pat. Nos. 5,782,103; 5,749,238; 6,018,959; 4,689,968; 5,809,794; 4,807,455; 4,617,804; 5,157,934; 5,259,210; 5,419,146; 5,632,154.
U.S. Pat. No. 6,141,981 discloses another microprocessor control of the expansion device to limit the mass flow rate of refrigerant through the compressor and avoid compressor overloading. More specifically, the compressor current draw, rather than a suction pressure or mass flow rate, is employed to adjust the refrigerant flow rate, by electrically adjusting the throttling valve. It is also well known to monitor compressor current draw and to turn the compressor off to avoid compressor overload, but such an approach eliminates all cooling or refrigeration effect.
While the above-described systems provide a way to control the mass flow rate of refrigerant to the compressor inlet, they are either additional mechanical devices or complex electronic devices. In addition while the spring actuated or electrical crankcase pressure regulator can be adjusted for a specific application, the MOP TVX cannot be field adjusted, and this lack of adjustability severely limits its flexibility.
Other devices to control vapor-compression systems include pressure switches or pressure monitoring devices that deactivate the compressor if the compressor discharge pressure is excessive. Likewise, using the relationship between a refrigerants saturation pressure and saturation temperature, the compressor could also be deactivated at a predetermined condenser refrigerant temperature.
U.S. Pat. No. 6,560,980 discloses a method and apparatus for controlling continuously-variable speed fans in both the evaporator and condenser fans of a refrigeration system to minimize the power consumed by the compressor and to minimize the unwanted heat added to the conditioned space due to the inefficiency of the evaporator fan. It includes providing a desired temperature for a conditioned space, measuring temperature at the inlet to the evaporator, and measuring temperature at the outlet to the evaporator. The method also includes calculating an actual temperature differential and adjusting evaporator fan speed based on the desired temperature differential and the actual temperature differential. This system is not designed to prevent compressor overloading but rather to maximize performance, by reducing heat added to the conditioned space by the fan and by reducing fan power consumption.
U.S. Pat. No. 5,782,101 discloses a device that prevents compressor overloading of a heat pump operating in heating mode, by cycling on/off or changing the speed of the evaporator fan, based on the pressure of the refrigerant at the compressor discharge rather than at the compressor inlet (crankcase pressure regulation usually controls the compressor inlet). The compressor inlet and outlet pressures are of course related, since a rise in the inlet pressure results in a rise in the outlet pressure, but they are typically not linearly related so a specific rise in the inlet pressure, will typically not create the same rise in outlet pressure. This patent also mentions that the refrigerant temperature instead of refrigerant pressure can be used. However, the compressor discharge temperature is determined from the compressor inlet enthalpy (inlet temperature and pressure), compressor discharge pressure and the work performed on the refrigerant by the compressor. Furthermore, the compressor inlet temperature is determined from the compressor inlet pressure and inlet enthalpy, with the evaporator design, evaporator temperature and evaporator superheat playing a role in the inlet enthalpy to the compressor. We also note that the compressor discharge temperature is not directly related to the ambient air temperature that is entering the evaporator, and FIG. 2 of the patent shows the pressure sensor 104 connected into the heat pump circuit directly after the compressor discharge 102 of compressor 101 and before the reversing valve 105.
We also consider it important to note that for a saturated refrigerant, the saturation pressure (evaporating or condensing refrigerant pressure) is directly related to saturation temperature, and therefore a refrigerant evaporating temperature (or refrigerant condensing temperature) can be substituted as the control variable instead of the refrigerant saturation pressure. However, the refrigerant entering or exiting the compressor is superheated, therefore the temperature of the refrigerant entering or exiting the compressor is not directly related to the pressure at these points, but is instead an independent variable as is well known.
Our invention proposes here to use the ambient air temperature (that enters the evaporator) and not any of the refrigerant pressures or refrigerant temperatures used by known devices. Thereby, we have developed an alternative method to control the suction line pressure and avoid compressor overloading so that complex mechanical or electronic devices are eliminated. Rather than add an additional device to actively control the flow of refrigerant and thus control the suction line pressure, we have recognized that the airside or, alternatively, water side heat transfer in the evaporator can be controlled to control the flow rate of evaporated refrigerant and therefore to control the suction side pressure.
In the case of airside heat transfer, there are alternative control methods such as the use of dampers to mask off a portion of the evaporator surface area, or even using a refrigerant bypass that short circuits portions of the evaporator to reduce the effective surface area of the evaporator. However, the currently preferred and simplest method is to modulate the speed or cycle (on/off) the evaporator blower. Modulating the speed of the fin, though feasible, is more complicated and costly than simply turning the fan on and off to adjust the average flow. Because of the thermal mass of the evaporator and the refrigerant contained in the evaporator, cycling the fan is just as effective, while being much simpler and less expensive to implement. Cycling the blower (on and off) will modulate the average heat transfer. While the compressor suction line pressure or the compressor current draw could be used to adjust the blower speed or to modulate the blower on and off to provide a lower effective heat transfer, we have discovered a better method of feedback control that is simpler, less expensive, and more reliable. We have discovered a method that has superior performance in actual operation and costs much less to implement, since air temperature measurement devices (such are thermistors, thermocouples, RTDs and the like) are much less expensive than refrigerant temperature sensors, refrigerant pressure sensors, or current sensors, and in many cases are already present in the system for temperature control or temperature monitoring. Also, they have the advantage over commonly used pressure sensors which are subject to drift and this requires frequent recalibration.
In the case of waterside heat transfer, the control can be, for example, a water bypass or a refrigerant bypass like that used in airside heat transfer described above. The simplest method is to modulate the speed or cycle (on/off) of an evaporator circulation pump/electric motor arrangement. Cycling in the case of a waterside heat transfer produces similar benefits as those described above for the airside transfer.
We have thus discovered that the ambient air temperature of the conditioned space, i.e., the temperature of the air entering the evaporator, can be used control the on/off operation (or fan speed) of the evaporator fan and still provide the compressor overload and crankcase pressure regulation function.
We have also discovered that if the temperature of the air entering the evaporator is not practical to monitor, then the temperature of the air entering the condenser can be used control the on/off operation (or fan speed) of the evaporator fan and provide the compressor overload and crankcase pressure regulation function.
We have also discovered that systems which use another type of suction side pressure regulation, such as the above-described embodiment a MOP TXV or crankcase pressure regulator can further benefit from the proposed evaporator blower cycling based on evaporator inlet air temperature (or condenser air inlet temperature), because our invention allows a multi-step control algorithm to be employed instead of simply regulating the crankcase pressure to below a single maximum suction side pressure. In fact a continuous change to the evaporator blower on/off duty cycle with temperature (continuous function) can be used instead of multiple step changes. In addition, the MOP TXV is pre-configured for a single maximum operating pressure and typically only available at two preset values. Also, the MOP TXV is not field adjustable, whereas our invention advantageously needs only a simple programming change in a control device to allow a different crankcase maximum operating condition to be easily implemented. The control device could even be programmed to learn and update the behavior, avoiding the need for manual programming changes.